The present invention relates to a lithographic apparatus, to device manufacturing methods using lithographic apparatus, and energy sensors.
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In device manufacturing methods using lithographic apparatus, it is important to ensure that the correct amount of energy (dose) is delivered to the substrate. An incorrect dose causes variation of line width and other imaging errors. Conversely, control of dose level can often be used for fine control of line width or critical dimension. To enable dose control, it is desirable to measure the power output of the radiation source, ideally as close as possible to the substrate. This is particularly important when a pulsed light source, such as an excimer laser, is used as the relationship between input to the light source is complex and may depend on history and factors not under direct control. Many lithographic apparatus divert a known fraction of the projection beam, e.g. using a partly silvered mirror, in the illumination system to an energy sensor. This therefore measures the power output of the radiation source and the effects of the illumination system upstream of the energy sensor during an exposure. Downstream effects can be predicted, based on calibration measurements taken using an energy sensor at substrate level when no exposure is taking place.
As well as dose control, various measurement and metrology processes carried out in lithographic apparatus require a measurement of the power of the radiation source. For example, in a process to align the substrate table to a mask, a sensor known as a transmission image sensor (TIS), which comprises a photodiode covered by a grating, mounted on the substrate table is scanned through the aerial image of a corresponding grating pattern on the mask. The output of the sensor is a periodically varying signal which, along with a position signal, can be used to determine the positional relationship of the substrate table and the mask pattern to a high degree of accuracy. When using a pulsed radiation source, it is desirable to remove the influence of any variation in source output from pulse to pulse. An additional sensor is provided adjacent the TIS to measure the pulse energy. The additional sensor comprises a photodiode connected to a RC network, or equivalent, which is sampled at a fixed time delay after the laser is fired. The resultant voltage measurement is used to normalize the signal from the TIS to eliminate source variations. However, this arrangement does not always give a correct measurement of the energy of a pulse.
. It is therefore desirable to provide an improved method for determining the pulse energy of a pulsed radiation beam.